The bacterial food incorporated into the healing agent is calcium lactate - a component of milk.

BBC: Key test for re-healable concrete

The group infused a plastic-like polymer with microscopic capsules containing a liquid healing agent.

BBC: Time to heal: The materials that repair themselves

The main challenge is to ensure the healing agent is robust enough to survive the mixing process.

BBC: Key test for re-healable concrete

Cracking open the material caused the capsules to rupture, releasing the healing agent.

BBC: Time to heal: The materials that repair themselves

Bond says this model had a shortcoming though, in that once the healing agent was used, it couldn't be replenished.

CNN: The emerging world of self-healing materials

Even if the healing agent adds 50% to the concrete cost, this makes up just 1-2% of the total construction cost.

BBC: Key test for re-healable concrete

In September HGS disclosed that its wound-healing agent, repifermin, had failed to heal venous ulcers in a trial of 352 patients.

FORBES: Beyond Talk

We have to produce the self-healing agent in huge quantities and we are starting to do outdoor tests, looking at different constructions, different types of concrete to see if this concept really works in practice.

BBC: Key test for re-healable concrete

To resolve this, researchers were focusing on developing a vascular system in which liquid could be circulated in a network of hollow fibers within the material, which would allow the healing agent to be restocked through an access point.

CNN: The emerging world of self-healing materials